Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to understand the fundamental quantum mechanical behaviour of few- and many-particle systems.


Energy shift and decay rate of the polaron as a function of temperature, displaying an intriguing behavior around the critical temperature.

Finite-temperature behavior of the Bose polaron

After our recent observation of the Bose polaron, we are aiming to understand the quasiparticle in more depth.

Here we consider a mobile impurity immersed in a Bose gas at finite temperature. Using perturbation theory valid for weak coupling between the impurity and the bosons, we derive analytical results for the energy and damping of the impurity for low and high temperatures, as well as for temperatures close to the critical temperature Tc for Bose-Einstein condensation. These results show that the properties of the impurity vary strongly with temperature. The energy exhibits an intriguing non-monotonic behavior close to Tc, and the damping rises sharply close to Tc. We finally discuss how these effects can be detected experimentally.

Read our manuscript in Physical Review A or on the arXiv.


Scattering properties of K and Rb, and the miscibility parameter. (a) Scattering lengths of K (dashed blue), Rb (dashed-dotted red) and between the two species (solid green). (b) The miscibility parameter ranges from miscible (>0) to immiscible (<0). The two dotted lines indicate immiscible (left) and miscible (right) parameters. (c-f) Examples of measurements.

Time-of-flight expansion of binary Bose-Einstein condensates at finite temperature

Ultracold quantum gases provide a unique setting to study the dynamics of interacting quantum systems, which is highly relevant for the understanding of the next generation of quantum devices. We study this scenario by investigating a multi-component system of 87Rb-39K Bose-Einstein condensates both theoretically and experimentally. Such multi-component systems can be characterized by their miscibility, where miscible components lead to a mixed ground state and immiscible components form a phase-separated state. Here we perform the first full simulation of the dynamical expansion of this system including both condensates and thermal clouds, which allows for a detailed comparison with experimental results. In particular we show that striking features emerge in time-of-flight for condensates with strong interspecies repulsion, even for systems which were separated in situ by a large gravitational sag. An analysis of the center of mass positions of the condensates after expansion yields qualitative agreement with the homogeneous criterion for phase-separation, but reveals no clear transition point between the mixed and the separated phases. Instead one can identify a transition region, for which the presence of a gravitational sag is found to be advantageous. Moreover we analyze the situation where only one component is condensed and show that the density distribution of the thermal component also show some distinct features. Our work sheds new light on the analysis of multi-component systems after time-of-flight and will guide future experiments on the detection of miscibility in these systems.

Read our manuscript on the arXiv.


Jan Arlt

New grant: Optical traps for quasiparticle quantum simulation

Jan Arlt has received a new grant from The Carlsberg Foundation: Optical traps for quasiparticle quantum simulation.


CQOM announcement picture

Center for Quantum Optics and Quantum Matter opens

The new Center for Quantum Optics and Quantum Matter (CQOM) was established and opened on November 8. The center brings together research from local experimental and theoretical groups who study diverse quantum phenomena, including the Ultracold Quantum Gases Group.

On the opening day, the different research directions were presented and discussed. Additionally, a CQOM talk was given by Nils Byg Jørgensen, and an invited talk was given by Richard Smith (ITAMP, Harvard-Smithsonian Center for Astrophysics and Harvard University).

Read more about the center at its home page.


Magnus with his price and the dean of Science and Technology at Aarhus University

Magnus wins the Deans Challenge

Magnus from the ultracold quantum gases group participated in this years annual case competition, Dean's Challenge. He had developed a rollator with automatic breaking and alarm system, which awarded him the first price in the HEALTH catagory. Congratulations to Magnus!

A news article in danish is available here.


The poster

Nils wins Poster Prize at CoQIPC 2017

The IST Austria recently hosted the Conference on Controllable Quantum Impurities in Physics and Chemistry (CoQIPC 2017). Nils went to the conference with his poster on the recent observation of bose polarons, and was awarded the Keysight Technologies Poster Prize of the poster competition. Congratulations!


Welcome Thomas

In this month, we welcome Thomas Guldager Skov, who starts as a Ph.D. in the MIX lab. He previously did a bachelors project with the MIX team, and is now ready to do full time research. Best of luck to Thomas!


Lars in his beloved lab

Goodbye Lars!

Lars Wacker found a job at Danish National Metrology Institute (DFM) in Copenhagen and will thus unfortunately leave our group. Lars did his PhD in the MIX lab and continued for a year and a half as a postdoctoral researcher. During his time here, he has been crucial in developing the MIX lab into its current great condition. Especially in the first production of KRb dual BECs, and in our few-body Efimov studies, he has made essential contributions. We wish him the best of luck in his future endeavours!


The portable setup of the rotating waveplate polarimeter

A portable rotating waveplate polarimeter

We describe the construction and performance of a polarimeter based on a quarter-wave plate rotated by a model airplane motor. The motor rotates at a high angular frequency of ω∼2π×160, which enables the polarimeter to monitor the polarization state of an incident beam of light in real-time. We show that a simple analysis of the polarimeter signal using the fast Fourier transform on a standard digital oscilloscope provides an excellent measure of the polarization state for many laboratory applications. The polarimeter is straightforward to construct, portable, and features a high-dynamic range, facilitating a wide range of optics laboratory tasks that require free-space or fiber-based polarization analysis. 

Read our manuscript in Review of Scientific Instruments.


Faraday image analysis

Sub-atom shot noise Faraday imaging of ultracold atom clouds

We have developed an imaging technique which can measure the atom number below the atom shot noise level. This work is closely related to our recent work on feedback stabilization of atom numbers. We use Faraday imaging which allows multiple images of the same cloud to be acquired. To describe the expected noise, we have developed a model based on photon shot noise and single atom loss. For clouds containing N~5×106 atoms, a precision more than a factor of two below the atom shot noise level is achieved.

Our manuscript on this work has been published in Journal of Physics B as part of a special issue on addressing quantum many-body problems with cold atoms and molecules.

Read the manuscript in Jour. Phys. B or on arXiv!



The Villum Foundation